Presently, in information processing apparatuses used in factories, power plants, transportation means such as vehicles, and computers, after the energy is consumed, the energy is discharged in the environment as waste heat. Thus, a thermoelectric generator capable of recovering the waste heat and converting the same into energy, is garnering attention.
A thermoelectric generator that uses Seebeck effect does not require working fluid or complex drive mechanisms as in the case of conventional thermoelectric conversion systems, and is capable of recovering waste heat from various systems and converting the same into electric power.
Particularly, strontium titanate (SrTiO3) which has been conventionally investigated in relation to application to ferroelectric devices, does not contain toxic or rare materials such as tellurium or bismuth as in the case of conventional thermoelectric generators, and is thought to be a promising material for a thermoelectric generator in view of the fact that potentially it may provide a relatively large value of 30 μW/cmK2 through 40 μW/cmK2 for the power factor PF, which is defined as S2σ (PF=S2σ). Here, S is a Seebeck coefficient and σ is electric conductivity of the thermoelectric generator. The power factor PF may be represented also as S2qnμ (PF=S2σ=S2qn μ), wherein n is a carrier density per unit volume, q is a carrier charge, and p is a carrier mobility. The power factor PF corresponds to the electric power that the thermoelectric generator may provide per unit temperature difference
Non-patent Document 1: Zide, J. M., et al. Phys. Rev. B74, 205335 (2006) pp 205335-1-205335-5
Non-patent Document 2: Kim, R., et al., J. Appl. Phys. 105, 034506 (2009)
While the system of SrTiO3 thus has the potential of providing the power factor PF of 35 μW/cmK2 through 40 μW/cmK2, it nevertheless has a problem, because of relatively large value of thermal conductivity κ of 11 W/mK for the case of a bulk crystal, in that the figure of merit ZT, defined asZT=PF·T/κ  (Eq 1)tends to take a limited value. In Equation 1, T stands for the absolute temperature, PF stands for the power factor of SrTiO3, and κ stands for the thermal conductivity of SrTiO3.
To obtain a large value for the figure of merit ZT in such a thermoelectric generator formed of SrTiO3, it is effective to increase the value of the Seebeck coefficient S and also the electric conductivity σ in view of the fact that the term of power factor PF is provided by Equation 2 below.PF=S2σ  (Eq 2)
Particularly, in the case the thermoelectric generator is used at room temperature or temperatures above the room temperature, it is desirable to increase the values of the Seebeck coefficient S and the electric conductivity σ, at the operational temperature of the thermoelectric generator such as the room temperature or the temperatures above the room temperature.